To change to single-user mode, use the telinit command, which instructs init to change runlevels:

# telinit 1

You can see from the table above that you can also shutdown or reboot the system in this manner. telinit 0 will halt the computer; telinit 6 will reboot the computer. When you issue the telinit command to change runlevels, a subset of the init scripts will run to either shutdown or startup system services.